I've been thinking about this a little bit and all I can say is "Yikes"! Doing this at the assembly language level essentially means verifying that every singl byte of 50K bytes has been executed at least once. Unless your application is some kind of calculation-only monster that does not interact/react to outside stimulus, I cannot conceive of a way that a static analysis (i.e., something that only looks at the code) could work. Here are a couple of ways it might be done.

1) Devise a sequence of tests to coax your program through "hunks" of code while running on an emulator with trace capture enabled, save the trace info to a text file and run it through a "tool" (you'd have to write this tool, I think) that checks off the locations executed against the address range(s) your application covers.

2) Again, perhaps working through chunks of code, in every code path, insert a "trace" call that records its execution (into memory, over the serial port, etc.) during runtime. Post-process the trace output for coverage. Tracing is common for larger systems. The last trace facility I designed was for a DASD Enterprise Disk system's i960-based Fibre Channel front end controller which had memory to spare for it. One might be hard-pressed to find memory in an 8051 system, so the serial solution might be preferred.